The reason I hear the most from people is that they're afraid to get attached, and I don't think it has much to do with not wanting to give them back, but I think people have a lot of anxiety about where the animal(s) will end up, and if they will have a great life.
Working in sheltering for almost 15 years, I've fostered a lot of kittens and a handful of dogs, but I've also seen a lot of animals returned and found stray--I still look back at the photos of all my fosters and hope they lived happy, full lives.
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